Method and apparatus for manufacture of a squirrel cage rotor

ABSTRACT

A method and apparatus is provided for manufacturing a squirrel cage winding assembly of a squirrel cage rotor from a pair of hollow cylindrical aluminum slugs wherein the pair of slugs are extruded into opposite ends of the winding slots of a rotor core. Conductor bars are formed in the slots by the extruded slug material and conductive end rings are formed integrally therewith by terminal portions of each slug.

United States Patent Savage 1 May 30, 1972 54 METHOD AND APPARATUS FOR2,125,970 8/1938 Waters ..29/598 MANUFACTURE OF A SQUIRREL 3,371,4103/1968 Gintovt... 29/598 X 3,469,309 9/1969 Sagalow ..29/598 CAGE ROTOR3,496,632 2/1970 Deming eta1.. ..29/598 [72] Inventor: Jack W. Savage,Centerville, Ohio 2,996,791 8/1961 Hicks ..29/598 I 3,191,270 6/1965Martin et a1. ..29/598 [73] Assigneet General Motors Corporation,Detroit,

Mich- Primary ExaminerTheron E. Condon [22] Filed: Apn 7, 1970 AssistantExaminer-Horace M. Culver Att0rneyE. W. Christen, C. R. Meland andRichard G. Stahr [21] App]. No.: 26,242

[57] ABSTRACT [52] U.S. Cl ..29/598, 72/354 A method and apparatus isprovided for manufacturing a [51] Int. Cl ..l-l02k l5/02, H02k 15/085 ql g win ing assembly of a squirrel cage rotor from a 58 Field of Search..29/598, 463, 470.5; 72 354 p of hollow cylindrical aluminum slugswherein the p of slugs are extruded into opposite ends of the windingslots of a 56] References Cited rotor core. Conductor bars are formed inthe slots by the extruded slug material and conductive end rings areformed integrally therewith by terminal portions of each slug.

3 Claims, 5 Drawing Figures PATENTEDMAY 30 m2 3, 665 593 [NV/5N7 OR.

BY JIM WSaI/ ge 51m WM ATTORNEY METHOD AND APPARATUS FOR MANUFACTURE OFA SQUIRREL CAGE ROTOR This invention relates to the manufacture ofdynamo-electric machines having squirrel cage rotors and moreparticularly to a method and apparatus for assembling a squirrel cagewinding assembly to a rotor core by an extrusion process.

In the manufacture of squirrel cage rotors for electric inductionmotors, various techniques have been employed for fabricating andassembling conductive end rings to the ends of the bar windings. In onemethod, solid bars of conductive material are inserted into the slots.Expensive machining and assembling apparatus are required and it isdifficult to fill the slots since they are usually skewed. In thismethod, end rings are usually cast on the opposite ends of the bars in aseparate manufacturing step. In a widely employed method, squirrel cagewindings are formed by die casting a conductive metal such as aluminumto a rotor core. Conductor bars and conductive end rings can be formedconcurrently in accordance with known casting techniques. For example,end ring molds having annular cavities are placed at the ends of therotor core to receive the molten casting material as it is cast into therotor slots so that the conductor bars and end rings are formed at thesame time.

Casting techniques are often troublesome because they involve handlingand working of hot molten metal in casting machines which are sometimeselaborate and expensive. One of the chief difficulties found in formingwindings by casting methods is maintaining the purity of the cast metal.Voids are produced by entrapment of air caused by turbulence during thecasting operation and by bubbles which are formed from dissolved gasessuch as hydrogen. Shrinkage cracks are also formed when the castingcools because of changes in density between the molten state and thesolid state.

The non-homogeneous castings produce undesirable electrical performancecharacteristics in the cast rotor windings. For example, a substantialdecrease in the electrical conductivity of the conductive metal has beennoted following casting. The castings are sometimes faulty and notuniform and must be rejected as falling production and reliabilityspecifications. Also, the cast rotor windings and end rings are oftenunbalanced and non-symmetrical so as to require extensive balancing andmachining operations to correct for the defects in the casting.

In the present invention. for a method and apparatus of manufacturing asquirrel cage rotor, a squirrel cage winding assembly is fabricated to alaminated rotor core having a plurality of winding slots by an extrusionprocess. The rotor core is mounted in an extrusion press assembly and apair of hollow cylindrical aluminum slugs are positioned adjacent the opposite ends thereof. The pairs of slugs are heated to an elevatedtemperature which is below the melting temperature and a hydraulicallyoperated ram extrudes the slugs into the opposite ends of the windingslots of the rotor core. The extruded material of each slug flows intothe slots and is bonded together therein to form integral conductorbars. Conductive end rings are formed by unextruded terminal portions ofeach of the pair of slugs which extend from the opposite ends of therotor core.

A primary object of this invention is to provide an improved method andapparatus for fabricating squirrel cage rotors wherein a pair of hollowconductive slugs are extruded into both ends of winding slots of a rotorcore to form conductor bar windings therein.

A further object of this invention is to provide an economical andreliable method and apparatus for fabricating conductor bar windings andintegral conductive end rings of a squirrel cage winding assembly whichhas reduced defects and increased purity by heating and extruding a pairof hollow cylindrical conductive slugs through both ends of the windingslots of a rotor core.

A still further object of this invention is to provide a method andapparatus for fabricating conductor bars and integral end rings to arotor core by extruding a pair of hollow cylindrical aluminum slugsthrough both ends of the winding slots of a rotor core so as to formintegral conductor bars therein and further wherein said conductor barsare provided integral with unextruded terminal portions of the hollowslugs so as to provide conductive end rings at opposite ends of therotor core.

Further objects and advantages of the present invention will be apparentfrom the following description, reference being had to the accompanyingdrawing in which a preferred embodiment of the apparatus and method ofoperation of the present invention is illustrated.

FIG. 1 is a sectional side elevational view of an extrusion pressassembly used in the present invention.

FIG. 2 is another view of the extrusion press assembly wherein the partsof the assembly have an intermediate operative position.

FIG. 3 is still another view of the extrusion press assembly of FIG. 1wherein the parts of the assembly have a position corresponding to thefinal operative position of the assembly.

FIG. 4 is a sectional end view of the extrusion press assembly of FIG. 2taken along lines 4-4 and looking in the direction of the arrows.

FIG. 5 is a perspective view, partially sectioned and broken away, of asquirrel cage rotor manufactured in accordance with the presentinvention.

Referring now to the drawings, FIG. 5 illustrates a typical squirrelcage rotor 10 made by the method and apparatus of the present inventionfor use in an induction motor type of dynamoelectric machine. Thesquirrel cage rotor 10 includes a cylindrical rotor core 12 formed by astack of circular disk laminations made of a magnetic iron material. Therotor core 12 includes a center opening 14 which is mounted to a motorshaft. The outer periphery includes a circumferential series of axiallyextending winding slots 16 disposed adjacent the outer diameter thereof.The winding slots 16 extend in a skewed or inclined relationship withrespect to the ends of the rotor core 12.

A squirrel cage winding assembly 18 is fabricated in accordance withthis invention to the rotor core 12. A series of bar windings isprovided by a plurality of conductor bars 20 which fill the windingslots 16. A pair of conductive end rings 22 and 24 are provided at theends of the conductor bars 20. Each of the end rings 22 and 24 isintegral with the respective ends of conductor bars 20 so as toelectrically interconnect and provide short circuiting conductive pathsat the ends of the bar windings provided by conductor bars 20.

FIGS. 1 through 4 illustrate an extrusion press assembly 30 forfabricating the squirrel cage winding assembly 18. A pair of metal slugs32 and 34, illustrated in FIG. 1 positioned within the extrusion pressassembly 30, provide the conductive material for forming the squirrelcage winding 18. The slugs 32 and 34 are made of an electricallyconductive material and are mounted adjacent the opposite ends of therotor core 12, as described more fully hereinbelow. The slugs 32 and 34have substantially identical predetermined hollow cylindrical shapes andare formed from a commercially available grade of substantially purealuminum. Other conductive metals such as copper may be used wheredifferent characteristics for the squirrel cage winding assembly 18 aredesired.

The component parts of the extrusion press assembly 30 include acontainer 38 provided by a pair of identical guide blocks 40 and 42. Theguide blocks are coaxially aligned and spaced apart by a rotor mountingblock 46. The guide blocks 40 and 42 and the mounting block 46 areillustrated in vertical alignment although it is to be understood theparts may be aligned horizontally, for example, if desired.

The mounting block 46 includes flat, parallel ends 50 and 52. The heightbetween the ends 50 and 52 is substantially equal to or slightly lessthan the height of the rotor core 12. A circular bore 54 extends axiallythrough the ends 50 and 52 providing a cylindrical wall which receives arotor core 12. The diameter of the cylindrical wall corresponds to thediameter of the rotor core 12 so that there is a force fit between theouter diameter of the rotor core 12 and the cylindrical wall.Accordingly, the rotor core 12 is placed in the mounting block 46 bybeing pressed into the bore 54.

The guide blocks 40 and 42 include axially extending annular openingsformed between outer and inner pairs of cylindrical walls 56 and 58 and62 and 64, respectively. These pairs of cylindrical walls defineidentical annular extrusion chambers 68 and 70, respectively. Theextrusion chambers 68 and 70 extend from the inner axial ends 74 and 76of the guide blocks 40 and 42 and are coaxially aligned with each otherand the winding slots 16 of the rotor core 12 when it is positioned inthe mounting block 46. The inner cylindrical walls 58 and 64 formmandrel portions 80 and 82 which are integral with the guide blocks 40and 42. The outer axial ends of the mandrel portions 80 and 82 areconnected with the guide blocks by bridging sections 84 as shown in FIG.4.

The radial spacing between the outer and inner pairs of cylindricalwalls 56 and 58 and 62 and 64, respectively, is slightly longer than theradial height of the end openings of the winding slots 16. The slugs 32and 34 are contained between the cylindrical walls of the extrusionchambers 68 and 70 so as to have a sliding fit.

Annular pistons 86 and 88 are disposed within the outer axial ends ofthe extrusion chambers 68 and 70 and are actuated by a pair of ramextensions 90 and 92, respectively. The inner and outer diameters of theannular pistons 86 and 88 have a substantially tight sliding fit betweenthe adjacent cylindrical walls 56 and 58 and 62 and 64, respectively.Each of the ram extensions 90 and 92 include slotted end openings 94 and96 which slide over the bridging sections 84 extending across the outeraxial ends of the extrusion chambers 68 and 70. The ram extensionsengage the outer axial ends of the pistons 86 and 88. and are connectedto a hydraulic press which applies the extrusion forces to the slugs 32and 34.

The inner axial ends 74 and 76 of the guide blocks 40 and 42 engage theflat ends 50 and 52, respectively, of the mounting block 46 and theouter peripheries of the ends of the rotor core 12. The axial ends ofthe mandrel portions 80 and 82 provide extensions of the ends 74 and 76for engaging the centers of the rotor core 12. Therefore, the inneraxial ends 74 and 76 provide axial mounting support for the ends of therotor core.

The extrusion press assembly 30 is held together by a clamping apparatusincluding a press, not shown, which clamps the inner axial ends 74 and76 of the pair of guide blocks 40 and 42 to the ends 50 and 52 of themounting block 46. The component parts are held in coaxial alignment andform sealed joints between the engaging surfaces of guide blocks 40 and42 and mounting block 46 and also between guide blocks 40 and 42 androtor core 12. Clamping of the extrusion press assembly 30 permitsfaster assembly and disassembly of the component parts than does the useof bolts extending through the component parts.

Referring now to the method of manufacture of my invention, theoperation of the extrusion press assembly 30 is described hereinafter.The rotor core 12 is fabricated prior to being mounted in the extrusionpress assembly 30 in accordance with the rotor core and apparatus forassembly disclosed in US. Pat. No. 3,1 10,831 to Zimmerle issued Nov.12, 1963 and assigned to the assignee of this invention. The rotor core12 is formed of a plurality of circular disk laminations having centerholes for forming the center opening 14 and circumferentially spacedslot openings disposed adjacent the outer periphery of the laminationsfor forming the winding I slots 16. The laminations further includeoffset segments 97 which interlock with adjacent laminations when thestack is axially compressed.

Prior to the laminations being compressed, the slot openings areoriented so that the circumferential series of winding slots 16 areformed at a skew angle of eighteen to twenty degrees relative to thecore longitudinal axis. Each lamination is rotated slightly relative tothe adjacent lamination so that the edges of the slot openings of onelamination extends slightly over the openings of an adjacent lamination.Thus, the winding slots 16 are formed so as to extend at an anglebetween the ends of the rotor core 12. The stack is then axiallycompressed so that the offset segments 97 are interlocked within acomplementary opening left by an offset segment of an adjacentlamination to form the laminations into an integral laminated assemblyforming the rotor core 12.

The dimensions of the rotor core 12 illustrated in FIG. 5 are describedhereinafter for purposes of illustrating the present invention. Therotor core has a diameter of approximately 2.625 inches, a height ofapproximately 1.375 inches and a circumferential series of thirtywinding slots 16. Each slot opening of the laminations has a radiallyextending height of approximately 0.22 inch and a width of 0.09 inch.Accordingly, the total volume of the winding slots 16 is approximately2.0 cubic inches.

After the rotor core 12 is assembled, it is mounted in the rotormounting block 46 by being pressed into circular bore 54. The mountingblock 46 and rotor core 12 are then ready for mounting in the extrusionpress assembly 30 and between the guide blocks 40 and 42.

The metal slugs 32 and 34 provided for positioning into the extrusionchambers 68 and 70 have a predetermined hollow cylindrical shape. Thesize of the metal slugs 32 and 34 is determined by the volume ofconductive material required to form conductive bars 20 and the endrings 22 and 24 of the squirrel cage winding assembly 18. Accordingly,this deter mines the cross-sectional area and the height of the slugs 32and 34. The volume of the slugs must be sufficient to fill theaforementioned volume of the winding slots 16 and also leavepredetermined unextruded terminal portions at the ends of each of theslugs 32 and 34 when the slots are filled. The terminal end portions aresubstantially identical and provide the conductive end rings 22 and 24having outer diameters of approximately 2.6 inches, inner diameters ofapproximately 1.5 inch and heights of approximately 0.5 inch.

A typical size of the hollow cylindrical metal slugs 32 and 34 used formanufacturing the rotor core 10 includes an outside cylindrical surfacehaving a diameter of approximately 2.5 inches and inner cylindricalsurface having a diameter of approximately 2.0 inches and a height ofapproximately 1.125 inch. The slugs are made of a commercially availablepure (99.75 percent minumum) aluminum material having a melting point ofapproximately l,250 F. Each slug weighs approximately 94 grams.

The slugs 32 and 34 are positioned into the inner axial openings of theextrusion chambers 68 and 70 of the guide blocks 40 and 42 and adjacentthe pistons 86 and 88. The guide blocks 40 and 42 are then clamped tothe mounting block 46 by a suitable clamping means such as a hydraulicpress. The ram extensions and 92 are aligned with the outer axial endsof the pistons 86 and 88 respectively and each is connected to a sourceof extrusion force as provided by a hydraulic press.

The extrusion press assembly 30 is heated to a temperature below themelting point of the slugs 32 and 34 within an enclosed heating chambermounted around the extrusion press assembly. A temperature range of1,000 to l,l50 F. has been found suitable for substantially reducing theuniaxial flow resistance or stress for extruding the slug material bydeforming it through the winding slots 16.

Pressure from the ram extensions 90 and 92 is applied to the pistons 88and 90, respectively, and against the outer axial ends of the slugs 32and 34. The temperature is maintained substantially within thetemperature range of 1,000 to 1,150" P. While the extrusion force isapplied from the ram extensions 90 and 92. It is important that thetemperature does not exceed this range since the incipient melting pointmay be reached due to heat developed during extrusion of the slugs 32and 34 through the winding slots 16.

The extrusion force applied by each ram extension 90 and 92 relative tothe end of the rotor core 12 is approximately 100,000 pounds. Asillustrated in FIG. 2, this causes the slugs 32 and 34 to be deformed bythe slot openings of the end laminations at opposite ends of the rotorcore 12 and flow into the winding slots 16. The edges of these end slotopenings effectively provide extrusion die surfaces. It has been foundthat the force of 100,000 pounds causes the slugs 32 and 34 to flowthrough the winding slots 16 at a rate of approximately three timesfaster than the rate at which the pistons 86 and 88 move within theextrusion chambers 68 and 70.

The extrusion forces of the ram extensions 90 and 92 continue until theextruded material of the two slugs 32 and 34 fills the winding slots 16and is pressed together at the centers thereof as shown in FIG. 3. Theextruded material of one slug is metallurgically bonded to the extrudedmaterial of the other slug so that an integral fusion bond 98 is formedtherebetween. This forms the conductor bars 20 from the extrudedmaterial of each slug extending through the winding slots 16. After thefusion bond 98 is formed, the extrusion is terminated by releasing theram extensions 90 and 92.

The aforementioned predetermined size of the slugs 32 and 34 willprovide unextruded terminal end portions at the outer axial endsthereof. The terminal end portions extend within the spaces 100 and 102of the inner axial openings of the extrusion chambers 68 and 70 betweenthe ends of the rotor core 12 and the inner axial ends of the pistons 86and 88. The unextruded terminal end portions of the slugs are radiallydeformed and widened slightly as they are pressed between thecylindrical walls of the extrusion chambers. These terminal end portionsprovide the conductive end rings 22 and 24 which are integral with theconductor bars 20 formed within the winding slots 16. The spaces 100 and102 provided at the inner axial ends of the extrusion chamber providethe size of conductive end rings noted hereinabove for the squirrel cagerotor 10. Accordingly, the entire squirrel cage winding assembly 18 isformed in the extrusion press assembly 30 in the final operativeposition of the pistons 86 and 88 as illustrated in FIG. 3.

The press assembly 30 is disassembled by removing the ram extensions 90and 92 and unclamping the guide blocks 40 and 42 from the ends of themounting block 46. The rotor core 12 is pressed from the annular bore 54of the rotor mounting block 46 and the completed rotor core is providedas shown in FIG. 5. If desired, the shape of the conductive end rings 22and 24 may be finished by machining steps or die forming operations sothat the conductive end rings have a finished configuration other thanthat provided by the method and apparatus as disclosed hereinabove.

The conductive end rings 22 and 24 and conductor bars formed by themethod and apparatus of this invention are substantially free of voids,cracks and imperfections which are found in the conductor bars and endrings formed by casting processes. Molten aluminum which is cast in arotor core normally cools at an uneven rate from the outer portion ofthe castings toward the center causing shrinkage and thermal stresscracking. Further defects are formed by voids caused by the turbulenceof the molten casting material during casting and dissolved gases whichform entrapped bubbles.

Undesirable electrical effects of the casting imperfections have beennoted by measuring the conductivity of the aluminum conductor bars andend rings. Conductivity measurements of a squirrel cage winding assemblyformed by extrusion was found to have a substantially higherconductivity than a winding assembly made by a casting process. Further,by visual and microscopic examination of cross-sections of an extrudedsquirrel cage winding assembly it was found to be substantially free ofdefects, cracks and voids when compared to those observed in theconductor bars and end rings formed by a casting process.

While the embodiment of the present invention as described hereinaboveconstitutes a preferred form, it is understood that other forms may beadopted without departing from the spirit of my invention.

What is claimed is as follows:

1. A method of manufacturing a squirrel cage rotor for fabricatingconductor bar windings interconnected at each end by conductive endrings to a rotor core having a circumferential series of axial windingslots, the steps comprising: positioning said rotor core in an extrusionpress having extrusion chambers communicating with opposite ends of saidaxial winding slots; placing hollow cylindrical slugs havingpredetermined heights and made of an electrically conductive material insaid extrusion chambers with one slug adjacent to and abutting each endof said rotor core; heating said slugs to a temperature below themelting point of said electrically conductive material to reduce theflow resistance of said slug; extruding said slugs from said extrusionchambers axially into said axial winding slots; terminating saidextruding when the extruded materials of said slugs are bonded togetherin said winding slots to form said conductor bar windings; and providingpredetermined unextruded terminal portions in each of said slugs therebyforming said conductive end rings integrally connected with the ends ofsaid conductor bar windings at the ends of said rotor core.

2. A method of manufacturing a squirrel cage rotor for fabricatingconductor bar windings interconnected at each end by conductive endrings to a rotor core having a circumferential series of axial windingslots, the steps comprising: assembling a stack of laminations havingcircumferentially spaced slot openings aligned at a predetermined skewangle to form a rotor core having a circumferential series of axialwinding slots; positioning said rotor core in an extrusion press havingextrusion chambers communicating with opposite ends of said axialwinding slots; providing a pair of hollow cylindrical slugs made ofaluminum and having predetermined sizes for filling said axial windingslots and for forming said conductive end rings from terminal portionsthereof; placing one of said pair of slugs in each of said extrusionchambers adjacent to and abutting each end of said rotor core; heatingsaid pair of slugs to a predetermined temperature range below themelting point of said aluminum material to reduce the flow resistance ofsaid slug; extruding said pair of slugs from said extrusion chambersaxially into said axial winding slots; terminating said extruding whenthe extruded aluminum materials of said pair of slugs are bondedtogether in said axial winding slots to form said conductor barwindings; and providing said terminal portions by unextruded ends ofeach of said pair of slugs thereby forming said conductive end ringsintegrally connected with the ends of said conductor bar windings at theends of said rotor core.

3. In a method of manufacturing a squirrel cage winding assembly for arotor core having a circumferential series of axial winding slots, thesteps comprising: positioning said rotor core in an extrusion presshaving extrusion chambers communicating with opposite ends of said axialwinding slots; placing a pair of hollow cylindrical slugs ofelectrically conductive material in said extrusion chambers with oneslug adjacent to and abutting each end of said rotor core; heating saidpair of hollow cylindrical slugs to a temperature below the meltingpoint of said electrically conductive material for reducing the flowresistance; extruding said pair of hollow cylindrical slugs from saidextrusion chambers axially into the opposite ends of said axial windingslots; and terminating said extruding when the extruded materials ofsaid slugs are pressed together within said axial winding slots so as toform said conductor bar windings in each of said axial winding slots.

1. A method of manufacturing a squirrel cage rotor for fabricatingconductor bar windings interconnected at each end by conductive endrings to a rotor core having a circumferential series of axial windingslots, the steps comprising: positioning said rotor core in an extrusionpress having extrusion chambers communicating with opposite ends of saidaxial winding slots; placing hollow cylindrical slugs havingpredetermined heights and made of an electrically conductive material insaid extrusion chambers with one slug adjacent to and abutting each endof said rotor core; heating said slugs to a temperature below themelting point of said electrically conductive material to reduce theflow resistance of said slug; extruding said slugs from said extrusionchambers axially into said axial winding slots; terminating saidextruding when the extruded materials of said slugs are bonded togetherin said winding slots to form said conductor bar windings; and providingpredetermined unextruded terminal portions in each of said slugs therebyforming said conductive end rings integrally connected with the ends ofsaid conductor bar windings at the ends of said rotor core.
 2. A methodof manufacturing a squirrel cage rotor for fabricating conductor barwindings interconnected at each end by conductive end rings to a rotorcore having a circumferential series of axial winding slots, the stepscomprising: assembling a stack of laminations having circumferentiallyspaced slot openings aligned at a predetermined skew angle to form arotor core having a circumferential series of axial winding slots;positioning said rotor core in an extrusion press having extrusionchambers communicating with opposite ends of said axial winding slots;providing a pair of hollow cylindrical slugs made of aluminum and havingpredetermined sizes for filling said axial winding slots and for formingsaid conductive end rings from terminal portions thereof; placing one ofsaid pair of slugs in each of said extrusion chambers adjacent to andabutting each end of said rotor core; heating said pair of slugs to apredetermined temperature range below the melting point of said aluminummaterial to reduce the flow resistance of said slug; extruding said pairof slugs from said extrusion chambers axially into said axial windingslots; terminating said extruding when the extruded aluminum materialsof said pair of slugs are bonded together in said axial winding slots toform said conductor bar windings; and providing said terminal portionsby unextruded ends of each of said pair of slugs thereby forming saidconductive end rings integrally connected with the ends of saidconductor bar windings at the ends of said rotor core.
 3. In a method ofmanufacturing a squirrel cage winding assembly for a rotor core having acircumferential series of axial winding slots, the steps comprising:positioning said rotor core in an extrusion press having extrusionchambers communicating with opposite ends of said axial winding slots;placing a pair of hollow cylindrical slugs of electrically conductivematerial in said extrusion chambers with one slug adjacent to andabutting each end of said rotor core; heating said pair of hollowcylindrical slugs to a temperature below the melting point of saidelectrically conductive material for reducing the flow resistance;extruding said pair of hollow cylindrical slugs from said extrusionchambers axially into the opposite ends of said axial winding slots; andterminating said extruding when the extruded materials of said slugs arepressed together within said axial winding slots so as to form saidconductor bar windings in each of said axial winding slots.